Probably a bad connection. It was discussed in an other thread somtimes ago, here is my reply ( I have actually tried this, and it works):
There is a simple test to check the operation of the masthead unit, you only need a voltmeter for that. It is described on Raymarine's site: https://raymarine.custhelp.com/app/a...ead-transducer
(You might not get the described results with the speed signal, az it is infact a square wave signal, the faster the wind blows, that higher its frequency. Depending on the meter you use, you might get alternating values or the described voltages.)
If you do not get these values, suspect the transducer or the cabling.